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1.
To determine whether fish impingement at an electric generating plant was significantly reduced during the operation of an air bubbler, impingement monitoring data was subjected to statistical analysis. Daily impingement rates were calculated for total fish collections and for three impinged species, and associated with water chemistry conditions and air curtain operation during impingement periods. Analysis for each species by season indicated that the air curtain was not an effective fish deterrent.  相似文献   

2.
A 16-month study of estuarine habitats in poly-, meso-, and oligohaline salinity regimes near Charleston Harbor assessed the distribution and abundance of megalopae and early crab stages of the blue crab,Callinectes sapidus. Blue crab were sampled with a plankton net and a cylindrical drop sampler. Blue crab were most abundant in plankton collections at night, accounting for 68% of the megalopae and over 88% of the juveniles collected in day and night tows combined. At night, densities of megalopae were greatest in surface samples, whereas densities in daylight collections were greater on the bottom. Juvenile densities were greatest on the bottom in both day and night collections, although catch rates at night were more variable than those of the megalopae. This suggests that megalopae, and possibly juvenile stages, experience a diel vertical migration. Results indicate that ingress to estuarine nursery areas occurs at the megalopal stage. Megalopal densities were highest at the polyhaline site, while juvenile blue crab were most abundant in the oligohaline area. Habitat utilization by juvenile blue crab was estimated using a cylindrical drop sampler and Venturi suction pump on three bottom types in the intertidal zone. Densities were greatest over the sandy-mud substrate, although catch rates were much lower than those reported for other geographical areas. These results suggest that juvenile blue crab do not occur in abundance on the marsh surface but remain on the creek bottom, possibly because creek physiography and large tidal amplitudes may restrict accessibility to the marsh surface.  相似文献   

3.
This study looks into the impacts of Sarawak Barrage (located in Kuching City, State of Sarawa Malaysia) flushing and flooding-in operations on upstream total suspended solids (TSS). Water samples were collected from three upstream stations on 17th, 18th and 19th of August 2000. During flushing, it was observed that TSS levels increased over a distance of 11 km upstream. Maximum TSS recorded 250 mg/L at 1.5 km, 120 mg/L at 6 km, and 85 mg/L at 11 km upstream. During flushing, TSS levels increased with depth, and upstream turbulence intensities were indirectly proportional to distance from barrage. During flooding-in operation, TSS decreased from 249 to 155 mg/L at 1.5 km near the bottom, while surface TSS decreased from 86 to 58 mg/L. Generally, during flooding-in operation, TSS increased from 90 to 116 mg/L between 6 and 11 km. During flooding-in operation, maximum bottom TSS recorded 216 mg/L at 6 km upstream. Differences between bottom and mid-depth TSS levels were relatively minimal ranging from 19 to 45 mg/L, whilst the maximum difference between mid-depth and surface TSS was as high as 78 mg/L. After 9 hours of gate closure, a well-mixed regime prevailed from 6 to 11 km upstream with TSS ranged from 35 to 47 mg/L at all the monitoring points regardless of depths.  相似文献   

4.
Beam trawls are a practical method for sampling vagile fauna from deep seagrass beds. The catches of fish and macroinvertebrates associated with Posidonia australis from two beam trawl designs were compared with the catch from a new design of beam trawl. Greater numbers of species and individuals of fish and macroinvertebrates were caught by the new trawl. Its catch also included almost the full range of species taken by the other two trawls. The new trawl was then trialed in the day and at night to compare differences in the catch of species between these times. Trawling at night yielded significantly more species and individuals of macroinvertebrates. In general, more fish were caught at night than during the day, but this was not the case at all sites, nor for all species. We emphasize the need for testing of sampling methods, and consideration of diel behavior patterns, when estimating abundance and species richness of macrofaunal seagrass communities.  相似文献   

5.
A persistent benthic nepheloid layer with high total suspended matter (TSM) and high total particulate surface area was observed in south-eastern Lake Michigan. The layer thickens from a few metres near the shelf-slope boundary to greater than 10 m at the base of the slope. When compared to the hypolimnion, TSM increases by a factor of 2-20 at 1 m above the bottom, the greatest increase detected at the slope-basin boundary. Sediment trap profiles within the nepheloid layer show that the particulate flux increases exponentially from about 10 m above the bottom to 1 m above the bottom, suggesting that a large fraction of the collected material came from resuspension. A nepheloid layer is created during the formation of the thermal bar and maintained during the stratified period, apparently through the action of weak but persistent currents. This layer is supplemented by lakeward transport of fine particles resuspended near the shelf-slope boundary due to impingement of the thermocline on the bottom, or during higher energy events.  相似文献   

6.
植物夜间蒸腾及其生态水文效应研究进展   总被引：5，自引：0，他引：5       下载免费PDF全文
传统理论认为植物在夜间由于气孔关闭而不会发生蒸腾现象,但是,越来越多的证据显示一些植物在夜间会开启气孔并且有蒸腾失水.夜间蒸腾通常为日间的5%~15%,在干旱和半干旱地区生态系统的夜间蒸腾能够占到日间蒸腾的30%以上.夜间蒸腾的控制具有相当的复杂性,涉及了不同层次的控制变量.除与植物固有的基因因素有关外,夜间蒸腾还受非生物因子的影响,如水汽压差、风速、土壤水分、大气CO2浓度和根区土壤温度等因素的影响,研究表明水汽压差是夜间蒸腾最重要的环境驱动因子.夜间蒸腾水分损失具有重要的生态水文意义,该过程可降低植物叶片水势,导致黎明前叶片水势和土壤水势不平衡,抑制水力再分配,影响生态水分平衡,增强植物的养分供应,影响植物生产力和生长状况.  相似文献   

7.
A common assumption with groundwater sampling is that low (<0.5 L/min) pumping rates during well purging and sampling captures primarily lateral flow from the formation through the well-screened interval at a depth coincident with the pump intake. However, if the intake is adjacent to a low hydraulic conductivity part of the screened formation, this scenario will induce vertical groundwater flow to the pump intake from parts of the screened interval with high hydraulic conductivity. Because less formation water will initially be captured during pumping, a substantial volume of water already in the well (preexisting screen water or screen storage) will be captured during this initial time until inflow from the high hydraulic conductivity part of the screened formation can travel vertically in the well to the pump intake. Therefore, the length of the time needed for adequate purging prior to sample collection (called optimal purge duration) is controlled by the in-well, vertical travel times. A preliminary, simple analytical model was used to provide information on the relation between purge duration and capture of formation water for different gross levels of heterogeneity (contrast between low and high hydraulic conductivity layers). The model was then used to compare these time–volume relations to purge data (pumping rates and drawdown) collected at several representative monitoring wells from multiple sites. Results showed that computation of time-dependent capture of formation water (as opposed to capture of preexisting screen water), which were based on vertical travel times in the well, compares favorably with the time required to achieve field parameter stabilization. If field parameter stabilization is an indicator of arrival time of formation water, which has been postulated, then in-well, vertical flow may be an important factor at wells where low-flow sampling is the sample method of choice.  相似文献   

8.
Depth-specific sampling at a single location was used to examine the vertical distribution of pediveliger larvae of bivalve mollusks in the York River, an estuary of Chesapeake Bay, Virginia. The water column at the sampling site was usually well mixed, lacking consistent temperature or salinity gradients for larvae. Four species showed strong vertical stratification when collected simultaneously at three depths. Pediveliger stage larvae of a clam (Cyrtopleura costata), an oyster (Crassostrea virginica), and a shipworm (Bankia gouldi) were most abundant near the benthos, and least abundant near the surface. A mussel (Geukensia demissa), showed the reverse trend, with most pediveliger larvae near the surface. Tidal stage had a slight effect on two species (C. costata andC. virginica), but only to increase the relative abundance of larvae near the benthos during flood tide. Otherwise, neither tidal phase nor light levels (night versus day) had detectable effects on distribution patterns. Sampling very close to the sediment-water interface provided no evidence that pediveliger larvae spent a significant proportion of their time in this location. While the behavior of precompetent bivalve larvae may tend to retain them within an estuary, that of competent-to-metamorphose pediveliger larvae does not appear to have that function. Pediveliger larvae may regulate their depth to best locate potential settlement substrate. Such an hypothesis is consistent with adult habitat zones of at least some of these species in Chesapeake Bay. Removing competent-to-settle larvae from analyses of larval distributions in estuaries will enhance or clarify depth distribution patterns observed for earlier-stage larvae.  相似文献   

9.
Diel activity cycles of toadfish and striped blennies were identified in the laboratory by means of mechanoelectric detector units. Data for both species were collected over 72-h periods. Daytime and nighttime activity rates were compared statistically. In addition, five 3-h segments (3 during the day and 2 at night) were selected for statistical comparison. both species are more active at night than during the day. The toadfish appears to be more strictly nocturnal than does the blenny, with the peak in its activity occurring in the 3-h segment following dusk. Blenny activity increases throughout the night, reaching a peak during the 3-h segment before dawn. It then ebbs to a low in the afternoon.  相似文献   

10.
The northern Gulf of Mexico (NGOMEX) was surveyed to examine the broad-scale spatial patterns and inter-relationships between hypoxia (<2?mg?L?1 dissolved oxygen) and zooplankton biovolume. We used an undulating towed body equipped with sensors for conductivity, temperature, depth, oxygen, fluorescence, and an optical plankton counter to sample water column structure, oxygen, and zooplankton at high spatial resolution (1?m??vertical; 0.25?C1?km??horizontal). We contrast the distribution of zooplankton during summer surveys with different freshwater input, stratification, and horizontal and vertical extent of bottom-water hypoxia. Bottom-water hypoxia did not appear to influence the total amount of zooplankton biomass present in the water column or the areal integration of zooplankton standing stock in the NGOMEX region surveyed. However, where there were hypoxic bottom waters, zooplankton shifted their vertical distribution to the upper water column during the day where they normally would reside in deeper and darker waters. When bottom waters were normoxic (>2?mg?L?1 dissolved oxygen), the daytime median depth of the water column zooplankton was on average 7?m deeper than the median depth of zooplankton in water columns with hypoxic bottom waters. A reduction in larger zooplankton when there were hypoxic bottom waters suggests that if zooplankton cannot migrate to deeper, darker water under hypoxic conditions, they may be more susceptible to size-selective predation by visual predators. Thus, habitat compression in the northern Gulf of Mexico due to hypoxic bottom water may have implications for trophic transfer by increasing the contact between predators and prey.  相似文献   

11.
A total of seventeen vertical profiles of ozone were obtained during an Indo-USSR collaborative experiment on ozonesonde intercomparison conducted at Thumba during March 1983. The vertical distribution of ozone was measured using rocket-borne, balloon-borne as well as ground-based instruments. Four different rocket ozonesondes from India and USSR and the balloon ozonesonde were used to makein situ observations of ozone concentrations in addition to the Dobson spectrophotometric observations of total ozone and Umkehr. The rocket and the balloon launchings were effected in three salvos and measurements were made at different times of the day as well as during night. The results of all these measurements are used to obtain a mean ozone vertical distribution over Thumba foT the spring equinoxial period. The mean profile shows the maximum ozone concentration at 27 km with a value of (3.86±0-52)×1012 molecules per cc. Comparison of this mean profile with available satellite data for the equatorial regions shows that, in general, the Thumba values are lower by 10–15% at altitudes below 40 km and larger at altitudes above 50 km compared to the satellite results. The data also show evidence for a day-to-day variability and a possible day-to-night variability in the ozone vertical distribution with the night-time values higher than the daytime values at all altitudes above 35 km and the difference is found to increase with the increasing altitude.  相似文献   

12.
Ocean currents impinging on topographic obstacles such as seamounts create a high level of variability in mesoscale physical oceanography. In the N Pacific, for example, the structure of the Kuroshio and its extension differ significantly E and W of the Emperor Seamount chain, and eddy fields detected downstream may be attributed to seamount effects. Nearfield effects of seamounts have been theoretically predicted for several decades but only recently has theory been confirmed by observation. Taylor columns, quasi-stationary eddies over seamounts, alter flow patterns and thus have impacts on both benthos on seamounts and on the biota in water overlying the seamount. SE Hancock Seamount, located at the N end of the Hawaiian Ridge (29°47′N; 179°04′E), has a summit depth of 265 m. This seamount is located near the subtropical front and is at the southerly extent of productive seamounts where trawl fisheries have existed in the past. The pelagic ecosystem in the upper 200 m over the seamount clearly differs from waters at control stations at distances of 10's of kilometers away as shown by plankton and midwater trawl hauls and hydroacoustic transects conducted during 1984 and 1985. Over the seamount, hydroacoustic transects show a significantly higher biomass of scatterers as compared to control stations. Sampling these scattering layers with small midwater trawls demonstrates high densities of a resident micronekton fauna dominated by the sternoptychid fish “Maurolicus muelleri” and the mysid “Gnathophausia longispina”; these taxa were virtually absent from the control stations, were oceanic micronekton, particularly larger forms, were generally in higher abundance than at the seamount stations. Similarly, ichthyoplankton abundance differs above the seamount and at reference stations. In summer sampling, larval fishes were less abundant over the seamount whereas in winter the abundance was greater there. The differences in distribution and abundance of both micronekton and ichthyoplankton are significant and consistently observed, suggesting that physical or biological processes at the seamount have important effects on the pelagic ecosystem. Hypotheses concerning current — topography interactions, exclusion of vertical migrators, and predation by resident micronekton and fishes can be used to explain the observed effects. Seamounts and other areas of complex topography are frequently sites of highly productive ecosystems; the S Emperor and N Hawaiian Ridge seamounts provide a good example, with a catch of approximately one million tons of boarfish in ten years. The interaction of ocean currents and complex topography may play an important role in this high productivity, as demonstrated in the high biomass of lower trophic levels in the seamount ecosystem. Interannual variability in the latitudinal position of the subtropical front and the strength of current flow over these seamounts may result in significant differences in mesoscale physical oceanography and therefore in the productivity of these ecosystems.  相似文献   

13.
This study was designed to examine effects of low dissolved oxygen on finfish, lobster, and squid under field conditions in western Long Island Sound. The relationship between bottom dissolved oxygen (DO) and catch was examined for effects on abundance, numbers of species, and mean length, for trawl sites throughout the sound. Examination of mean catch per tow, and species number per tow, showed that both abundance and diversity decreased markedly with bottom DO, with dramatic declines at sites with DO<2 mg l?1. Of 18 species examined, 15 were found to occur with greater frequency at sites with DO>3 mg l?1 compared to sites with <2 mg l?1, and three of these species occurred significantly less frequency at a DO of 2–3 mg l?1. Trawl samples taken in the Narrows west of Greenwich, an area which chronically experiences summer hypoxia (DO<3 mg l?1) consistently yielded below average species number during hypoxic events. However, when DO was>3 mg l?1, 13 of 14 tows had above average species number. Abundance of five common species at sites>3 mg l?1 DO were significantly higher than at sites <3 mg l?1 DO in this area. Lobster was the only species not showing this DO threshold. Patterns in abundance vs DO for squid, bluefish, and butterfish suggest that these species are among the most sensitive to hypoxia. Only one of the four examined species, winter flounder, showed a decrease in mean length with DO. Scup, bluefish, and lobster lengths did not vary significantly with respect to dissolved oxygen.  相似文献   

14.
天基GPS探测可以提供分布全球、高精度、高垂直分辨率的电离层观测资料, 弥补地基GPS探测受地域限制、垂直分辨率低的问题.利用COSMIC掩星2008—2011年数据,网格化统计分析了太阳活动处于不同水平下的电子密度空间分布变化,以期为分析电离层电子密度的空间扰动变化提供参考背景.研究表明,电子密度存在明显赤道异常和威德尔海异常现象,并发现随太阳活动性增强,在250~350 km高度范围电子密度赤道异常现象扩展到中纬地区,在300~500 km高度范围威德尔海异常现象不仅发生在南半球夏季,春秋季电子密度夜间值比白天也显著增强;在250~500 km高度范围,电子密度存在明显的威德尔海异常现象,并随着太阳活动性增强,异常区域地理范围扩宽.   相似文献   

15.
It has been observed that storms in early fall can result in top-to-bottom mixing of Chesapeake Bay. A three-dimensional, time-dependent circulation model is used to examine this destratification process for September 1983, when extensive current and hydrographic data were available. The model bay is forced at the surface by observed hourly winds, at the ocean boundary by observed hourly surface and bottom salinities and sea level fluctuations, and at the head by observed daily discharges for a 28-d period. A second-moment, turbulence-closure submodel, with no adjustments from previous applications to its requisite coefficients, is used to calculate the vertical turbulence mixing coefficients. Comparisons with data inside the model domain indicate relative errors of 7% to 14% for sea level, 7% to 35% for current, and 11% to 21% for salinity. The tidal portion of the spectrum is modeled better than the subtidal portion. The model is used to examine both the mechanisms of wind mixing and the temporal and spatial distribution of vertical mixing within the estuary. Wind-driven internal shear is shown to be a more effective mechanism of inducing destratification than turbulence generated at the surface. The model is also used to show that the vertical temperature inversion which occurs in the fall does not affect the timing of the destratification as much as its completeness. The distribution of mid-depth vertical mixing shows highly variable values in the mid-bay region, where wind-induced mixing is dominant. This suggests that the source of oxygen to mid-bay bottom waters is similarly variable. Vertical turbulence mixing coefficients of 10?2 cm2 s?1 (background) to 103 cm2 s?1 were needed to simulate the September period, indicating the need for time-variable mixing in models of dissolved and suspended estuarine constituents.  相似文献   

16.
采用RNG湍流模型对动水中含污染物冲击射流滞止点下游区域的横向浓度分布特征进行了详细的数值研究,并与相应的实验结果进行了比较。分析了下游过渡区内横向高浓度聚集区的形成机理和扩散特性。计算结果表明,冲击效应和横流绕流对冲击射流下游过渡区内的横向浓度分布具有重要的影响。当流速比相对较小时,在靠近对称面以及底层壁射流与环境横流的横向边界附近出现较为明显的横向高浓度聚集区,计算结果表明,冲击效应产生的横向逆压梯度以及横流绕流导致的Scarf涡结构对横向高浓度聚集区的形成起主导作用。  相似文献   

17.
We report results of bottom temperature monitoring of 2003–2004 in the deepwater South Baikal basin (Lake Baikal) near active gas-fluid methane vents at lake depths of 1020 and 1350 m. Sediments and water temperatures were measured using an autonomous temperature recorder designed at the Institute of Geophysics (Novosibirsk). Experiments implied short-duration recording and pioneering continuous 350 day-long monitoring near the Staryi vent. Measurements within a 1 m thick layer above and below the bottom showed notable variations in water (up to 0.07 °C) and sediment temperatures and in geothermal gradient. The long temperature records include a relatively steady period (mid-June 2003-early February 2004) with smooth temperature variations (especially in sediments) and two transient unsteady periods. The steady season is the best time for heat flow studies in the South Baikal basin. The 0.04–0.05 °C drop in bottom water temperature during the unsteady periods may result from intrusion of cold surface water. A positive temperature anomaly of ∼0.04 °C recorded in April 2003 may be caused, among other reasons, by active gas venting.  相似文献   

18.
The relatively slow flow and exchange of Carr Inlet water with the main basin of Puget Sound, Washington, favor eutrophication. To study Carr Inlet’s circulation, the Model-measurement Integration Experiment in Estuary Dynamics (MIXED) was conducted in March–May 2003, spanning the spring bloom. From observations and numerical simulations the circulation was decomposed into tidal and subtidal components; the former was dominated by the M2 tide, the latter by atmospheric forcing. Near the surface, the subtidal velocity was correlated with wind. At mid depths, the subtidal velocity was organized into vertical bands arising from internal waves excited by wind forcing of the water surface. The tidal flow was more strongly steered by local bathymetry and weaker in peak magnitudes than the subtidal flow, yet it contributed more mechanical energy to the inlet. Tidal eddies reduce exchange of water through the inlet’s entrances. Numerical simulations with the Princeton Ocean Model recreated many observed features, including the three-layer vertical structure of outflow at the surface and bottom and inflow at mid depth, the mid-depth subtidal response to the wind, and characteristics of the tide. While the model produced greater subtidal flow magnitudes at depth and differences in the phase of the M2 tide compared to observations, overall the case study provided support for more comprehensive simulations of Puget Sound in the future.  相似文献   

19.
Hypoxia/anoxia in bottom waters of the Rappahannock River, a tributary estuary of Chesapeake Bay, was observed to persist throughout the summer in the deep basin near the river mouth; periodic reoxygenation of bottom water occurred on the shallower sill at the river mouth. The reoxygenation events were closely related to spring tide mixing. The dissolved oxygen (DO) in surface waters was always near or at the saturation level, while that of bottom waters exhibited a characteristic spatial pattern. The bottom DO decreased upriver from river mouth, reaching a minimum upriver of the deepest point of the river and increasing as the water becaume shallower further upriver. A model was formulated to describe the longitudinal distribution of DO in bottom waters. The model is based on Lagrangian concept—following a water parcel as it travels upriver along the estuarine bottom. The model successfully describes the characteristic distribution of DO and also explains the shifting of the minimum DO location in response to spring-neap cycling. A diagnostic study with the model provided insight into relationships between the bottom DO and the competing factors that contribute to the DO budget of bottom waters. The study reveals that both oxygen demand, either benthic or water column demand, and vertical mixing have a promounced effect on the severity of hypoxia in bottom waters of an estary. However, it is the vertical mixing which controls the longitudinal location of the minimum DO. The strength of gravitational circulation is also shown to affect the occurrence of hypoxia. An estuary with stronger circulation tends to have less chance for hypoxia to occur. The initial DO deficit of bottom water entering an estuary has a strong effect on DO concentration near the river mouth, but its effect diminishes in the upriver direction.  相似文献   

20.
Biogeochemical cycles of carbon, sulfur, and free oxygen in a microbial mat   总被引：11，自引：0，他引：11  
Complete budgets for carbon and oxygen have been constructed for cyanobacterial mats dominated by Microcoleus chthonoplastes from the evaporating ponds of a salt works located in Guerrero Negro, Baja California Sur, Mexico. Included in the budget are measured rates of O2 production, sulfate reduction, and elemental exchange across the mat/brine interface, day and night, at various temperatures and times of the year. We infer from this data the various sinks for O2, as well as the sources of carbon for primary production. To summarize, although seasonal variability exists, a major percentage of the O2 produced during the day did not diffuse out of the mat but was used within the mat to oxidize both organic carbon and the sulfide produced by sulfate reduction. At night, most of the O2 that diffused into the mat was used to oxidize sulfide, with O2 respiration of minor importance. During the day, the internal mat processes of sulfate reduction and O2 respiration generated as much or more inorganic carbon (DIC) for primary production as diffusion into the mat. Also, oxygenic photosynthesis was the most important process of carbon fixation, although anoxygenic photosynthesis may have been important at low light levels during some times of the year. At night, the DIC lost from the mat was mostly from sulfate reduction. Elemental fluxes across the mat/brine interface indicated that carbon with an oxidation state of greater than zero was taken up by the mat during the day and liberated from the mat at night. Overall, carbon with an average oxidation state of near zero accumulated in the mat. Both carbon fixation and carbon oxidation rates varied with temperature by a similar amount. These mats are thus closely coupled systems where rapid rates of photosynthesis both require and fuel rapid rates of heterotrophic carbon oxidation.  相似文献   

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